1. Field of the Invention
The present invention relates to amplifiers used at the end of an amplification signal path and, more particularly, to amplifiers used for connecting to a load device having an electrical characteristic exhibiting resonances.
An amplifier used at the end of the signal path in an amplification portion of an analog system is usually connected to a load device which is to be driven by signals larger in magnitude but more or less following the signal waveform provided at the beginning of the particular amplification portion of the system upon which amplification is performed. For a simple load device, this usually causes no problem beyond having adequate power available at the amplifier output to drive this load device as directed by the amplified signal. For more complex load devices, however, further measures may be needed because of the complex electrical impedance characteristics that such devices can exhibit.
A typical complex load device would be a speaker, ranging from a hearing aid speaker to a public address system speaker. Such speakers have electrical characteristics which depend not only on the electrical materials present, but also the mechanical construction of the load device and the acoustical surroundings in which such a device is placed. That is, the electrical impedance characteristic of the device will reflect therein mechanical structural resonances modified by, and perhaps added to by, resonances arising as a result of the acoustical impedance faced by the speaker. The occurrance of such resonances leads to uneven performance across the frequency range in which the system is to perform.
Improvements in these resonance characteristics to reduce differences between different parts of the operating frequency spectrum of the system are usually obtained through the use of negative feedback from the amplifier output to a point earlier along the amplifier signal path. Voltage feedback is commonly used, and in some systems has been supplemented by current feedback. The latter, however, has been obtained through sensing the current passing through the load which reduces the range of output voltage over which the load can be driven. Thus, an output amplifier is desired, particularly in low supply voltage situations, which can control resonances in the load device without limiting the voltage range over which such a device may be driven.